Objectives:To estimate the proportions of US infants who were breastfed exclusively for 6 months, according to characteristics of the mother, child, and household environment, and to compare associations between those characteristics and exclusive breastfeeding with associations between those characteristics and breastfeeding initiation.Methods:Data were obtained from the 2007 National Survey of Children's Health, a nationally representative, cross-sectional survey. Multivariate logistic regression was used to calculate the adjusted odds ratios for breastfeeding among all infants and for breastfeeding exclusively for 6 months among infants who had initiated breastfeeding. All analyses were limited to children aged 6 months through 5 years for whom breastfeeding data were available (N = 25 197).Results:Of the nearly 75% of children in the study who had ever been breastfed, 16.8% had been breastfed exclusively for 6 months. Non-Hispanic black children were significantly less likely to have ever been breastfed compared with their non-Hispanic white counterparts (adjusted odds ratio: 0.54 [95% confidence interval: 0.44-0.66]). However, no significant differences in the odds of exclusive breastfeeding according to race were observed. Children with birth weights of <1500 g were most likely to have ever been breastfed and least likely to have been breastfed exclusively. Maternal age was significantly associated with exclusive breastfeeding; however, maternal age was not associated with breastfeeding initiation.Conclusions:In the United States, the prevalence of exclusive breastfeeding for 6 months remains low among those who initiate breastfeeding. Factors associated with breastfeeding exclusively for 6 months differ from those associated with breastfeeding initiation.